An amplifying system with reduced feedback

ABSTRACT

An amplifying system, particularly for application in which the input and output transducers are closely coupled acoustically, for example, hand-held power megaphones and the like, as a result of which feedback howl may occur, in which an amplifier stage thereof is constructed for class B operation and is provided with a capacitor in the input circuit thereof, arranged to be charged by an input signal and apply a reverse bias to said input circuit operative, in the presence of a sustained feedback howl, to effect a reduction in the gain of such stage and the amplifier sufficient to cut off such howl.

United States Patent Queen [4 1 Mar. 21, 1972 [541 AMPLIFYING SYSTEMWITH 5 a 3,536,836 10/1970 Pfeiffer ..179/1 vc REDUCED FEEDBACK3,187,268 6/1965 Bauer et al... ..l79/l 3,324,406 6/1967 Fristoe..330/l5 [72] Inventor: Daniel L. Queen, Chicago, Ill. 7 [73] Assignee:Chamberlain Manufacturing Corporation, 'f' Exami'fer xathleen ClaffyChicago, In. Assistant Examiner-Douglas W. Olms AttorneyHill, Sherman,Meroni, Gross & Simpson [22] Filed: Aug. 11, 1969 [21 Appl. No.: 849,016[57] ABSTRACT An amplifying system, particularly for application inwhich the input and output transducers are closely coupled acoustically,(gl. ..l7 9/l FS, 179/ l k: for example, hand-held power megaphones andthe like, as a l 58] Field Search 330/15 76 129 139 179 A result ofwhich feedback howl may occur, in which an amplifi- 179 Fs 325/462 A2 1. er stage thereof is constructed for class B operation and isprovided with a capacitor in the input circuit thereof, arranged to becharged by an input signal and apply a reverse bias to said [56]References cued input circuit operative, in the presence of a sustainedfeed- UNITED ATES PATENTS back howl, to effect a reduction in the gainof such stage and the amplifier sufficient to cut off such howl.3,531,728 9/1970 Visher ..330/l5 3,534,268 10/1970 Mazziota ..325/475 10Claims, 4 Drawing Figures PATENTEUhMZI I972 3.651.266

flV/PdTT lNPl/ 7' INVEN'I UR jazgziglji pa e 7M w a WTTORNEYS ANAMPLIFYING SYSTEM WITH REDUCED FEEDBACK BACKGROUND OF THE INVENTIONSpeech amplification systems of the small, self-contained type, such asthe hand-held, so called power megaphone have numerous advantages whichhave contributed to their increasing popularity. Among these are thesmall size of the complete self-contained unit, capable of being readilyheld in the hand and in effect forming a megaphone, the self-containedpower supply eliminating connecting lines, etc., as well as thetransport and operation of relatively large bulky amplifiers and thelike. However, such amplifying systems, because of their compact size,have one undesirable characteristic. As the microphone associated withthe amplifier input and the loudspeaker associated with the amplifieroutput are contained in a unitary housing, in relatively closeproximity, a relatively short in-phase feedback oftentimes occur. Thisis particularly true with respect to a howlback which oftentimes takesplace when no audio signal is being fed into the microphone. Obviously,the use of transistors in such an amplifier is desirable from the pointof size, power consumption, etc., and as such types of speechreinforcing systems are normally used in locations of relatively largeexpanse as for example outdoors or in a relatively large enclosure, suchamplifiers are normally of relatively high gain. As a result it iscomparatively easy to have the amplifier gain set at a value which willresult in an in-phase feedback factor greater than unity and theproduction of an undesired howl.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to theproduction of an amplifier system, of the type referred to, which is sodesigned that upon application of a signal the gain of the amplifierwill be automatically reduced by a desired predetermined amount,sufficient in the presence of an undesirable in-phase feedback have toeliminate the same.

This is accomplished in the example of the invention illustrated by theutilization of a class B transistor amplifier as one of the stages ofthe amplifying system, such stage being illustrated herein as apush-pull amplifier employing two transistors, which is biased for classB operation whereby the stage is biased in the exponential gain regionnear the origin of the transistor gain characteristic. Disposed in theinput circuit is a suitable storage element such as a capacitor which isadapted to be charged when a signal is applied to such input circuit andwhich is operative to provide a reverse bias to such input circuitwhereby the gain of the amplifier stage is thereby reduced.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein likereference characters indicate like or corresponding parts, and wherein apreferred form of the invention is illustrated, which, as will beapparent therefrom and from the following description, may be capable ofvarious immaterial variations and modifications without departing fromthe spirit and scope of the novel concepts of the disclosure:

FIG. 1 is a block diagram of an amplifying system of the type hereinvolved;

FIG. 2 is a circuit diagram of a push-pull amplifier stage constructedin accordance with the present invention;

FIG. 3 is a graph illustrating the gain characteristic of the amplifyingstage illustrated in FIG. 2; and

FIG. 4 is a graph illustrating the gain-time relation of the amplifierillustrated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION As illustrated merelydiagrammatically in FIG. 1, amplification systems of the general typehere involved comprise an amplifier l to the input of which is connecteda microphone 2 and to the output of which is connected a loudspeaker 3,the three components being suitably mounted in a housing 4 or the like,indicated in broken lines in FIG. 1, which forms a self-containedunitary structure which is sufficiently small in size that itmay behand-held, as for example by means of a suitable handle 5. In theembodiment illustrated, the housing 4 is depicted as being generallyconical in shape having the microphone 2 disposed at the small endthereof and the large end being open and communicating with theloudspeaker 3. Thus the user may hold the device by means of the handle5 in the same manner as a megaphone, with the microphone 2 positioned toenable the user to speak directly into the same. The physical usagethereof thus is similar to that of the usual megaphone but enables apower amplification of the sound received at the input end thereof.

The amplifier 1 may consist of any desired number of amplifier stages,consistent with the requirements of the particular unit and FIG. 2illustrates one such stage, that depicted being illustrated as being theoutput stageof the amplifier with its output connected to the speaker 3.

The amplifier stage of FIG. 2 is illustrated as being of the push-pulltype having two transistors T1 and T2 disposed in a grounded-emittertype circuit with the respective bases of such transistors beingconnected to the ends of the center tapped secondary winding 6 of aninput transformer 7 having a primary winding 8.

The emitters of the transistors T1 and T2 are connected together andadapted to be connected to the positive side of a battery 9 or otherpower source.

The collectors of the transistors T1 and T2, in the embodimentillustrated, are connected to the respective ends of the primary winding10 of a transformer 11, illustrated as comprising an output transformer,the secondary winding 12 of which is operatively connected to theloudspeaker 3. Obviously, the amplifier stage illustrated in FIG. 2 maycomprise the output'stage of the amplifier 1, as illustrated, or maycomprise another stage of such amplifier. Likewise, if desirable, theinput transformer could be omitted and the amplifying stage capacitivelycoupled with the preceding stage, in conjunction with a phase inverter,as is well known in the art. Also, if the amplifier stage is to beemployed as an intermediate stage and coupled to the input of afollowing pushpull stage, the collectors of the transformers T1 and T2may be capacitively coupled to the respective inputs of such followingstage.

The transistors TI and T2 are provided with a bias voltage determined byresistances l3 and 14, the values of which are so selected that therespective transistors are biased for class 8 operation. Referring toFIG. 3 which illustrates the gain characteristics t, it will be notedthat the gain initially increases rather slowly and then makes arelatively sharp increase forming a bend or knee in the characteristiccurve and the bias of the amplifier stage thus is set in the exponentialgain region near the origin of the transistor gain characteristicwhereby the stage will have a characteristic curve correspondingsubstantially to that of FIG. 3.

Connected between the center tap of the winding 6 and the emitters ofthe transistors T1 and T2 is a capacitor 15. The latter thus is sodisposed that in the presence of an input signal the capacitor willcharge, whereby a reverse bias voltage results in the base circuits ofthe respective transistors T1 and T2, operative to effect a reduction inthe gain of the amplifier. This is graphically illustrated in FIG. 4which illustrates the du/dt slopeat the bend or knee of the curveillustrated in FIG. 3.

It will be apparent that in the operation of the circuit illustrated inFIG. 2 the amplifier stage will function as a class B amplifier wherebythe collector current of the respective transistors will in the absenceof an applied signal at the input, be approximately or close to zero;upon the appearance of a signal at the input, the amplifier gaininitially will increase, in accordance will class B operation, and thenas the capacitor 15 charges, the gain will decrease from its initiallevel to a somewhat lower level, as indicated in FIG. 4. While this willhave no noticeable effect on the operation of the system with respect tothe desired signals to be amplified, the gain reduction is sufficient,in the absence of an input signal, to cut off any sustained howlback asthe gain will be reduced to a value sufficient to reduce the in-phasefeedback factor necessary to sustain the howlback.

Theinvention thus enables the production of an amplifying system of thetype here involved which will pennit an increase in the amplifier gain,before the creation of a howlback, beyond that heretofore possible.

It will be appreciated from the above description that the invention isnot directed to an automatic level control which is intended toautomatically maintain substantially uniform output level with varyinginput levels, but rather to provide an increase in the usable gainbefore a feedback howl will occur, and more particularly to effect anautomatic cutoff of a sustained howlback.

l claim as my invention: 7

l. A threshold controlling circuit, particularly for use in a speechreinforcement system to reduce feedback howl in the absence of an audiosignal, comprising an amplifier stage having an electronic amplifierdevice having a pair of electrodes including an output electrode,between which electrodes a current may flow, an input electrodecooperable with said pair of electrodes by means of which such currentflow between said pair of electrodes may be controlled, an input circuitconnected to said control electrode and an output circuit connected tosaid output electrode, means connected to said input circuit foroperatively connecting thereto a signal at said stage input, means forapplying a biasing voltage to said input circuit of a magnitude toeffect class B operation of said stage, and means connected to saidinput circuit, responsive to an input signal thereat,-for producing areverse bias voltage at such input circuit increasing the gain thresholdof said amplifier stage, operative to effect a reduction in thelow-level gain of said amplifier, said reverse bias-producing meanscomprising a capacitor so disposed in said input circuit that it willcharge upon application of such an input signal and apply a reverse biasvoltage to such input circuit.

2. An amplifier stage as defined in claim 1, wherein said electronicdevice comprises a transistor having base, emitter and collectorelectrodes, the base electrode comprising said control electrode andsaid collector electrode comprising said output electrode.

3. An amplifier stage as defined in claim 2, wherein said reversebias-producing means comprises a capacitor so disposed in said basecircuit that it will charge upon application of such an input signal andapply a reverse bias voltage to such base circuit.

4. An amplifier stage as defined in claim 2, wherein said stagecomprises in further combination a second transistor, said transistorsbeing circuited for push-pull operation, and

said reverse bias-producing means being connected in a common portion ofthe base circuits thereof.

5. An amplifier stage as defined in claim 4, wherein said reversebias-producing means comprises a capacitor so disposed in said commonportion of the base circuits of said transistors that it will chargeupon application of such an input signal and apply a reverse bias to therespective base circuits of said transistors.

6. An amplifier stage as defined in claim'5, wherein an input couplingtransformer is provided having a center-tapped secondary winding, theends of which are connected to the respective bases of said transistors,said bias voltage for efi'ecting class B operation being applied to saidcenter tap, one side of said capacitor also being connected to saidcenter tap.

7. An amplifier stage as defined in claim 6, wherein said bias an out utelectrical-audio transducer respective] connected to san amplifier inputand output, the distance etween and relation of said transducers beingsuch that an in-phase feed-- back may occur during operation of saidsystem, said amplifier including a class B transistor stage circuitedfor push-pull operation, having base, emitter and collector circuits,means connected to the base circuit of said stage for operativelyconducting an input signal thereto, means for applying a voltage to saidbase circuits of an magnitude to effect a class B operation of saidstage, and means connected to said base circuit, responsive to an inputsignal thereat, for producing a reverse bias voltage at such basecircuit increasing the gain threshold of said stage, operative in thepresence of sustained feedback howl, to efiect a reduction in thelow-level gain of said amplifier sufficient to cut off such howl, saidreverse biasproducing means comprising a capacitor so disposed in saidcommon portion of the bias circuits of said transistors that it willcharge upon application of such an input signal and apply a reverse biasto the respective base circuits of said transistors.

9. An amplifying system as defined in claim 8, wherein an input couplingtransformer is provided having a center-tapped secondary winding, theends of which are connected to the respective bases of said transistors,said bias voltage for effecting class B operation being applied to saidcenter tap, one side of said capacitor also being connected to saidcenter tap.

10. An amplifying system as defined in claim 9, wherein said biasvoltage for effecting class B operation is derived from a voltagedivider having an intermediate point thereof connected to said centertap, said capacitor being connected across a portion of said voltagedivider.

1. A threshold controlling circuit, particularly for use in a speechreinforcement system to reduce feedback howl in the absence of an audiosignal, comprising an amplifier stage having an electronic amplifierdevice having a pair of electrodes including an output electrode,between which electrodes a current may flow, an input electrodecooperable with said pair of electrodes by means of which such currentflow between said pair of electrodes may be controlled, an input circuitconnected to said control electrode and an output circuit connected tosaid output electrode, means connected to said input circuit foroperatively connecting thereto a signal at said stage input, means forapplying a biasing voltage to said input circuit of a magnitude toeffect class B operation of said stage, and means connected to saidinput circuit, responsive to an input signal thereat, for producing areverse bias voltage at such input circuit increasing the gain thresholdof said amplifier stage, operative to effect a reduction in thelow-level gain of said amplifier, said reverse bias-producing meanscomprising a capacitor so disposed in said input circuit that it willcharge upon application of such an input signal and apply a reverse biasvoltage to such input circuit.
 2. An amplifier stage as defined in claim1, wherein said electronic device comprises a transistor having base,emitter and collector electrodes, the base electrode comprising saidcontrol electrode and said collector electrode comprising said outputelectrode.
 3. An amplifier stage as defined in claim 2, wherein saidreverse bias-producing means comprises a capacitor so disposed in saidbase circuit that it will charge upon application of such an inputsignal and apply a reverse bias voltage to such base cirCuit.
 4. Anamplifier stage as defined in claim 2, wherein said stage comprises infurther combination a second transistor, said transistors beingcircuited for push-pull operation, and said reverse bias-producing meansbeing connected in a common portion of the base circuits thereof.
 5. Anamplifier stage as defined in claim 4, wherein said reversebias-producing means comprises a capacitor so disposed in said commonportion of the base circuits of said transistors that it will chargeupon application of such an input signal and apply a reverse bias to therespective base circuits of said transistors.
 6. An amplifier stage asdefined in claim 5, wherein an input coupling transformer is providedhaving a center-tapped secondary winding, the ends of which areconnected to the respective bases of said transistors, said bias voltagefor effecting class B operation being applied to said center tap, oneside of said capacitor also being connected to said center tap.
 7. Anamplifier stage as defined in claim 6, wherein said bias voltage foreffecting class B operation is derived from a voltage divider having anintermediate point thereof connected to said center tap, said capacitorbeing connected across a portion of said voltage divider.
 8. Anamplifying system comprising an amplifier having an input and an output,an input audio-electrical transducer and an output electrical-audiotransducer respectively connected to said amplifier input and output,the distance between and relation of said transducers being such that anin-phase feedback may occur during operation of said system, saidamplifier including a class B transistor stage circuited for push-pulloperation, having base, emitter and collector circuits, means connectedto the base circuit of said stage for operatively conducting an inputsignal thereto, means for applying a voltage to said base circuits of anmagnitude to effect a class B operation of said stage, and meansconnected to said base circuit, responsive to an input signal thereat,for producing a reverse bias voltage at such base circuit increasing thegain threshold of said stage, operative in the presence of sustainedfeedback howl, to effect a reduction in the low-level gain of saidamplifier sufficient to cut off such howl, said reverse bias-producingmeans comprising a capacitor so disposed in said common portion of thebias circuits of said transistors that it will charge upon applicationof such an input signal and apply a reverse bias to the respective basecircuits of said transistors.
 9. An amplifying system as defined inclaim 8, wherein an input coupling transformer is provided having acenter-tapped secondary winding, the ends of which are connected to therespective bases of said transistors, said bias voltage for effectingclass B operation being applied to said center tap, one side of saidcapacitor also being connected to said center tap.
 10. An amplifyingsystem as defined in claim 9, wherein said bias voltage for effectingclass B operation is derived from a voltage divider having anintermediate point thereof connected to said center tap, said capacitorbeing connected across a portion of said voltage divider.